Fragrancenet. com is a great site. I've ordered from them several times & feel that you don't need to worry about ordering from them.
As for Ebay, you have to be careful, but it's a great place to get often substantial discounts on perfume. I buy most of my perfume there, either from a business or the partially used bottles of an individual.
I had the misfortune of buying three fake perfumes in my seven years on Ebay: Allien, Chanel No. 5 EDP, & Chanel Allure Sensuelle. With the Alien, it was easy to tell, because I had just smelled it at Sephora, plus on the box it indicated that it was, 90ml/6.8oz! With the Chanels, I hadn't smelled either scent previously and at that time, I didn't even know that fake perfumes were sold on Ebay! So it was long after I bought them that I figured out they were fakes.
I would definitely stay away from a Chanel or Dior that starts at 99 cents and used a stock photo in the listing. Also, it's usually the 100ml size that's faked, so you can worry less if it's a smaller size. Also, with Chanels, the batch code on the box and bottle should be the same. The code is lasered discreetly on the back of the bottle, so you can ask for that if it's a listing for just the bottle. I'm afraid I don't know the particulars of how Dior "marks" their packaging, but I think it's been discussed here.
But you don't need to worry about fragrancenet! Enjoy your perfumes when they arrive!
I agree with Adrien02 that sometimes people will falsely claim they got a fake perfume. With feedback, I think you have to be the judge of what sounds like fair feedback and what may be unfair from someone who is almost impossible to please.